1. Field of the Invention
The present invention relates to an optical color printer assembly for printing a color image on photosensitive recording paper by exposing three colors, namely red, green, and blue, by means of liquid crystal shutter elements which open and close when subjected to image data.
2. Description of the Related Art
Generally, in an optical printer assembly for printing an image on photographic recording paper by exposure to light by means of liquid crystal shutter elements which open and close when subjected to image data, a number of liquid crystal cells (liquid crystal shutter elements) are arranged two-dimensionally on a liquid crystal panel, as in the liquid crystal printer assembly disclosed in Japanese Patent Laid-Open No. HEI 4-241965, for example. Japanese Patent Laid-Open No. HEI 2-169271, for example, is also conventionally known as a means of exposing in the same position the three colors, namely red, green, and blue, required in an optical color printer assembly (a liquid crystal color printer assembly).
That is, in the liquid crystal color printer assembly disclosed in the above laid-open patent three types of monochrome light sources R, G, and B are used in which red light, green light, or blue light constituting a light source radiate downwards, and these three types of monochrome light sources are arranged in three positions spaced in a direction of motion of photosensitive paper (a photographic recording medium) disposed below so that the three types of monochrome light sources meet in a straight line at the focus below. Liquid crystal shutters for trichromatic separation consisting of three types of liquid crystal shutters for R, G and B opening sequentially at predetermined intervals in response to color change signals are disposed on a plane crossing the optical path of the monochrome light at a lower position and are arranged so as to be spaced in the direction of motion of the photo-sensitive paper so as to allow only red light, green light, or blue light to pass through. A number of liquid crystal shutter elements opening for the exact exposure time required at a given shutter element position for a given color in response to image signals synchronized with the opening time of each of the R, G and B liquid crystal shutters are positioned at the height of the above-mentioned focus of the light, being disposed in a line perpendicular to the direction of motion of the photosensitive paper. The photosensitive paper is disposed on a plane below these shutter elements and is fed continuously and at constant speed in a direction perpendicular to the above-mentioned line so as to be synchronized with the transmission time for one line of R, G and B image signals.
In the conventional construction, the above-mentioned liquid crystal shutters for trichromatic separation receive color change signals and, for example, each opens for a predetermined time in RGB order, sequentially allowing the passage of only red, green, or blue light towards the above-mentioned rectilinear focus along mutually different paths. The rectilinear liquid crystal shutter elements (shutter arrays) receive RGB image signals synchronized with the transit time for each of the above colored lights and open for the exact exposure time required at a given shutter element position for a given color, sequentially allowing the passage of red, green, or blue light to the photosensitive body in a line dot pattern.
At that time, because the photosensitive body is conveyed continuously and at constant speed by a conveying means in a direction perpendicular to the line so as to be synchronized with the transmission time for one line of trichromatic RGB image signals and, as explained above, the incident paths of R, G, and B into the liquid crystal shutter array are different, one line of trichromatic overlaid exposure is performed by first exposing red light, for example, in a line dot pattern slightly ahead of the above-mentioned linear focus in the direction of motion of the photosensitive paper, next exposing green light on the line dot patterned previously exposed portion of the photosensitive paper which has been conveyed to be directly below the linear focus during the previous exposure time, and further exposing the previously photosensitized portion to blue light aft of the linear focus.
However, because the conventional assembly uses three types of monochrome light sources and two layers of liquid crystal shutters, there is a large number of parts, leading to enlargement of the assembly. Furthermore, because two layers of liquid crystal shutters are used, the permeability of the light from the light sources deteriorates and the quantity of light is reduced, and thus one problem has been the necessity to extend exposure time to account for insufficient light, leading to a reduction in printing speed. Furthermore, because the liquid crystal elements have incident angle dependency making the permeability of light vary according to the angle of incidence of the light, another problem has been the difficulty in adjusting the angle of incidence of the three colors of light, facilitating dislocation and blurring of the focus on the image.